I have one of those, and I have to say that I am really impressed with it.
Firstly Behringer instruction manuals are very good. Not only do they explain their product very well, they also give you a broad over view of compression in general, and how to get certain kinds of sounds or effects (in terms of how it affects your sound) from the unit.
Secondly you can use it in mono, but daisy chain both channels to give a layered compression, or to compress in two different ways, or whatever. If you go in stereo, you can just set the channel one settings to control both channels (very useful), without having to adjust channel two settings. It is a complex bit of kit to use, there are so many variables that I honestly find it a bit overwhelming.
I have used mine in a couple of different applications. Firstly and most successfully whilst practicing in a small room, running into a desk. I am not a technical adept when it comes to this sort of thing, but I seem to remember that I was set up something like: guitar > jmp1 (with behringer virtualiser in the effects loop) stereo out (possibly the speaker emulator)> behringer mixer> PA. I got the best sounds I have ever had from any instrument in any application from this setup. I am not sure that I could do it again personally, but our singer was really into recording, and he insisted on me buying this bit of kit, and once he set it up it rocked. We did A-B tests with and without it, and it was very clear that just that one unit made a huge amount of difference.
I have since used it in a similar rack setup going into the power stage only of a peavey mace (ugh! I know). It wasn't as good. It may be that it was not the right application for the unit, or it was set up badly (by me), or it is just a horrible amp. Probably a bit of all three.
I got the best (most noticeable) results using it with digital, and transitor preamps (or modellors). It really adds superb valve warmth to your sound. I would say if you are doing that kind of recording, then it is a really useful thing to have in your armoury.
I think that it really excels in that it has a really transparent sound, you have to A-B it to understand where it is helping you, but it is without a doubt a quality bit of kit. It doesn't impose anything on your tone at all.
I don't know about the dbx at all, but if you are looking for a studio type of compressor, then I can recommend this one.